George Lucas has already addressed this subject at length.
Back when he was making the first trilogy, that was his master plan: a 9-movie series. Once he was done with the prequel trilogy, he would take another break and begin making the sequel trilogy. He outlined this scenario in the famous Time magazine issue with Darth Vader on the cover back in 1980.
However, he became sidetracked by several other projects, and the 16-year gap between movies was much longer than he had originally planned. As a result, he said that he won't be making the sequel films, and he went on to say that he won't grant another director/producer permission to take over in order to complete the series, either. The Star Wars universe is his intellectual property, and he is extremely protective.